Nullability exercise

sum.kt

fun sum1(a: Int?, b: Int?): Int {
  b ?: return 0

  return if (a != null) {
    a + b
  } else {
    5 + b
  }
}

fun sum2(a: Int?, b: Int?): Int {
  val first = a ?: 5
  val second = b ?: return 0
  return first + second
}

fun sum3(a: Int?, b: Int?): Int = when {
  b == null -> 0
  a == null -> 5 + b
  else -> a + b
}